Method, apparatus, and system for encoding and decoding image using LM chroma prediction

ABSTRACT

Disclosed are methods, apparatuses, and systems for encoding and decoding an image. The present invention provides an intra prediction unit that receives an input image, calculates a correlation between a Luma area block and Chroma area block in the input image in intra prediction, removes high frequency ingredients by low-pass filtering an encoded luma pixel based on the calculated correlation, and generates a prediction block by applying an LM chroma mode, which is an extended chroma mode technique, to the luma pixel removed therefrom the high frequency ingredients.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/193,406, filed Nov. 16, 2018; which is a continuation of U.S.application Ser. No. 15/101,780, filed Jun. 3, 2016, now U.S. Pat. No.10,158,882, issued Dec. 18, 2018; which is the U.S. national stageapplication of International Patent Application No. PCT/KR2014/009222,filed Sep. 30, 2014; which claims the benefit under 35 U.S.C. § 119 ofKorean Patent Application No. 10-2013-0149075, filed Dec. 3, 2013, eachof which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to methods of encoding and decodingimage, apparatus and system therefor, and more particularly, to methods,apparatus, and a system for encoding and decoding an image for improvingefficiencies of predictions between color planes of a RGB image in highefficiency video coding (HEVC).

BACKGROUND ART

Following the standardized high efficiency video coding (HEVC), a HEVCrange extension is currently being standardized for developing anextended technique for supporting 4:2:2 and 4:4:4 color samplings andYUV images and RGB images corresponding to 10 bit, 12 bit, and 14 bitdepths. Here, unlike an YUV image, a RGB image exhibits highcorrelations between respective color planes R, G, and B. Based on thesame, an extended chroma mode (referred to hereinafter as ‘ECM’) hasbeen suggested by J. Kim at a recent MPEG standardization conference (J.Kim, “RCE1: The performance of extended chroma mode for non 4:2:0format”, JCTVC-M0097, 13th JCT-VC Meeting, Incheon, Korea, April 2013)

The ECM suggested by J. Kim is a technique for predicting pixel valuesin a chroma (or B, R) area with reference to an encoded luma (or G)pixel in case of an intra prediction for chroma (U, V; B, R in the RGBcoding) pixel. The algorithm employs a technique suggested by J. Chenduring the standardization of the HEVC, but is not employed by the HEVC(J. Chen, V. Seregin, W-J Han, J. Kim, B. Jeon, “CE6.a.4: chroma intraprediction by reconstructed luma samples”, JCTVC-E266, 5thMeeting,Geneva, Switzerland, March 2011). However, unlike the YUV 4:2:0 formatof the HEVC, the RGB 4:4:4 format exhibits high correlations betweencolor planes. Therefore, if the technique suggested by J. Chen ismodified in correspondence to the RGB 4:4:4 format instead of the YUVformat and applied to the RGB 4:4:4 format, compression efficiency maybe significantly improved. In other words, a high coding gain may beobtained. Therefore, the technique may be applied to the HEVC rangeextension.

However, as a result of analysis of a large number of RGB images, acommon RGB image exhibits low correlations between color planes at highfrequency areas of the color planes. In other words, due to lowcorrelations between color planes at high frequency areas, highfrequency ingredients for luma area may interfere chroma pixelprediction during an application of the ECM, thereby deterioratingcompression efficiency.

TABLE 1 G/B G/R Test sequence LL LH HL HH LL LH HL HH Traffic 0.9260.634 0.614 0.549 0.963 0.837 0.810 0.777 Kimono1 0.948 −0.105 0.136−0.008 0.977 −0.011 0.303 0.045 EBULupoCandlelight 0.820 0.859 0.8690.899 0.904 0.944 0.929 0.940 EBURainFruits 0.909 0.972 0.970 0.9550.892 0.971 0.969 0.956 VenueVu 0.720 0.885 0.853 0.553 0.597 0.8400.604 0.229 DucksAndLegs 0.906 0.227 −0.090 −0.154 0.963 0.238 0.125−0.186 OldTownCress 0.981 0.456 0.265 0.025 0.984 0.617 0.466 0.243ParkScene 0.919 0.247 0.443 0.117 0.972 0.406 0.462 0.126 Overall 0.8910.522 0.508 0.367 0.907 0.605 0.584 0.391

Table 1 shows results of calculating correlations between four frequencybands between color planes (G-B and G-R) by using first frames of 8 RGBexperimental images currently used in the HEVC range extension and showscorrelations between color planes corresponding to respective frequencybands in HEVC range extension RGB experimental images. Table 1 ispublished in “Color plane interpolation using alternating projections,”(B. K. Gunturk, Y. Altunbasak, and R. M. Mersereau), IEEE Trans. VideoProcessing, Vol. 11, No. 9, pp. 997-1013, September 2002. The techniquedisclosed therein can analyze frequencies of a RGB image based onmathematical analysis of correlations between color planes R, G, and B,where each correlation has a range from −1 to 1. The higher thecorrelation indicates the corresponding color planes having the highercorrelation to each other.

In Table 1, the LL frequency band indicates low frequency ingredients ofan image, the LH and HL frequency bands indicate horizontal and verticaledge ingredients of the image, and the HH frequency band indicatesdiagonal edge ingredients of the image, that is, high frequencyingredients. Except an EBULupoCandlelight image and an EBURainFruitsimage, images generally exhibit high correlations in low frequency areasLL and low correlations in high frequency areas HH. Referring to Table1, RGB images exhibit high correlations (0.90 average) in LL frequencybands and low correlations (0.38 average) in HH frequency bands. Asshown in Table 1, high frequency ingredients having low correlationscompared to the other frequency ingredients having generally highcorrelations may deteriorate compression efficiency when pixel valuesbetween color planes are predicted by using the ECM.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

The present invention provides methods of encoding and decoding an imagefor improving efficiency of predicting pixel values between color planesby filtering high frequency ingredients exhibiting low correlationsbetween the color planes.

The present invention also provides apparatuses for encoding anddecoding an image for accomplishing the technical goal.

The present invention also provides a system for encoding and decodingan image for accomplishing the technical goal.

Technical Solution

According to an aspect of the present invention, there is provided animage encoding apparatus including an intra prediction unit thatreceives an input image, calculates a correlation between a Luma areaand Chroma areas during intra prediction, removes high frequencyingredients by low-pass filtering an encoded luma pixel based on thecalculated correlation, and generates a prediction block by predictingthe chroma pixel by applying an LM chroma mode, which is an extendedchroma mode technique to the luma pixel removed therefrom the highfrequency ingredients.

The intra prediction unit includes a chroma predicting unit, whichincludes a plurality of pre-set mode prediction units including a LMchroma mode prediction unit that generates the prediction block byapplying the LM chroma mode to the encoded luma pixel, wherein the modeprediction unit individually perform intra prediction to generate aplurality of prediction blocks; a rate-distortion optimizing unit, whichreceives the plurality of prediction blocks and determines an optimalprediction block from among the plurality of prediction blocks accordingto pre-set rules; and a mode selecting unit, which sets up a flagcorresponding to a mode for generating the optimal prediction block fromamong the plurality of modes and outputs the flag together with theoptimal prediction block.

The LM chroma mode prediction unit includes a correlation calculatingunit, which calculates a correlation between the Luma area block and theChroma area block and determines whether the calculated correlation issmaller than a pre-set threshold value; a low pass filter, which, if thecalculated correlation is smaller than the pre-set threshold value,low-pass filters the luma pixel; and an extended chroma prediction unit,which, if the calculated correlation is equal to or greater than thepre-set threshold value, generates the prediction block by applying theLM chroma mode to the luma pixel applied by the correlation calculatingunit and, if the calculated correlation is smaller than the pre-setthreshold value, generates the prediction block by applying the LMchroma mode to the luma pixel low-pass filtered by the low pass filter.

According to another aspect of the present invention, there is providedan image decoding apparatus including an intra prediction unit thatreceives a bitstream, analyzes a flag included in the bitstream duringan intra prediction, removes high frequency ingredients by selectivelylow pass filtering a luma pixel based on a set value of a correlationflag included in the analyzed flag, and generates a prediction blockregarding Chroma areas by applying an LM chroma mode, which is anextended chroma mode technique to the luma pixel removed therefrom thehigh frequency ingredients.

The intra prediction unit includes a flag analyzing unit, which analyzesa flag set to the bitstream; a mode selecting unit, which selects a modefor performing intra prediction from among a plurality of pre-set modesincluding the LM chroma mode based on the analyzed flag; and a chromapredicting unit, which includes a plurality of pre-set mode predictionunits including a LM chroma mode prediction unit that generates theprediction block by applying the LM chroma mode to the luma pixel,wherein the mode prediction unit individually perform intra predictionto generate a plurality of prediction blocks.

The LM chroma mode prediction unit includes a correlation flagdetermining unit, which determines whether a correlation flag is set tothe flag analyzed by the flag analyzing unit; a low pass filter, which,if the correlation flag is set, low-pass filters the luma pixel; and anextended chroma prediction unit, which, if no correlation flag is set,generates the prediction block by applying the LM chroma mode withregard to the luma pixel applied by the correlation calculating unitand, if the correlation flag is set, generates the prediction block byapplying the LM chroma mode to the luma pixel low-pass filtered by thelow pass filter.

If the correlation flag is set, the chroma predicting unit low-passfilters the luma pixel by using the low pass filter, calculates thelow-pass filtered the luma pixel according to Equation G_(recon)^(lpf)|x,y|=0.5 G_(recon)|x,y|+0.125 G_(recon)|x+1,y|+0.125G_(recon)|x−1,y|0.125 G_(recon)|x,y+1|+0.125 G_(recon)|x,y−1|, appliesthe LM chroma mode to the low-pass filtered luma pixel, and generatesthe prediction block according to an Equation C_(pred)[x,y]=αG_(recon)^(lpf)[x,y]+β (here, G_(recon) ^(lpf) denotes a low pass filtered lumapixel, and α and β are weights inferred by using the luma pixel at samelocations as pixel values around the Chroma blocks).

According to another aspect of the present invention, there is providedan image encoding method employed by an image encoding apparatus thatreceives an input image, encodes the input image based on intraprediction, and outputs a bitstream, the method including calculating acorrelation between a Luma area block and a Chroma area block in theinput image during intra prediction; if the correlation is smaller thana pre-set threshold value, removing, by the image encoding apparatus,high frequency ingredients by low-pass filtering the encoded luma pixel;if the correlation is smaller than the pre-set threshold value,generating a prediction block by applying an LM chroma mode, which is anextended chroma mode technique, with regard to the low-pass filteredluma pixel or, if the correlation is equal to or greater than thepre-set threshold value, generating a prediction block by applying theLM chroma mode to the luma pixel that are not low-pass filtered; and,generating a bitstream of a RGB image exhibiting high correlationsbetween color planes by using the prediction block and outputting thebitstream.

According to another aspect of the present invention, there is providedan image decoding method employed by an image encoding apparatus thatreceives a bitstream, encodes the bitstream based on intra prediction,and outputs a restored image, the method including receiving, by theimage decoding apparatus, the bitstream and obtaining a Luma area block;obtaining and analyzing a flag included in the bitstream and selecting amode for performing intra prediction from among a plurality of pre-setmodes; if a mode selected based on the analyzed flag is a LM chromamode, analyzing the setting of a correlation flag included in the flag;if the correlation flag is set, removing high frequency ingredients ofthe luma pixel by low-pass filtering the luma pixel; if the correlationflag is set, generating a prediction block by applying the LM chromamode, which is an extended chroma mode technique, with regard to thelow-pass filtered luma pixel and, if no correlation flag is set,generating a prediction block by applying the LM chroma mode to the lumapixel that are not low-pass filtered; and outputting the resconstructedimage by using the prediction block.

According to another aspect of the present invention, there is providedan imaging system including an image encoding apparatus that receives aninput image, calculates a correlation between a Luma area block and aChroma area block in the input image during intra prediction, removeshigh frequency ingredients by low-pass filtering the encoded luma pixelbased on the calculated correlation, generates a prediction block byapplying an LM chroma mode, which is an extended chroma mode technique,with regard to the low-pass filtered the luma pixel removed therefromthe high frequency ingredients, and generates and outputs a bitstream ofa RGB image exhibiting high correlations between color planes by usingthe prediction block; and an image decoding apparatus that receives thebitstream and obtaining a Luma area block, analyzes a flag included inthe bitstream, removes high frequency ingredients of the luma pixel byselectively low-pass filtering the luma pixel based on the setting of acorrelation flag of the analyzed flag, generates a prediction block byapplying the LM chroma mode, which is an extended chroma mode technique,to the luma pixel removed therefrom the high frequency ingredients, andoutputs the reconstructed image by using the prediction block.

Advantageous Effects

Therefore, according to methods, apparatuses, and a system for encodingand decoding an image according to the present invention, during intraprediction, a correlation between color planes is calculated in anextended chroma mode, low-pass filtering is selectively applied withregard to a Luma area block, and a prediction block is generated byperforming a LM chroma mode. Therefore, encoding or decoding may beperformed in the LM chroma mode after low-pass filtering is selectivelyperformed based on a correlation, and thus coding efficiency may beimproved.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing in-screen correlations between highfrequency areas of color planes in an image;

FIG. 2 is a schematic block diagram showing an image encoding apparatusaccording to the present invention;

FIG. 3 is a diagram showing an example of the intra prediction unit ofFIG. 2;

FIG. 4 shows a detailed configuration of the LM chroma mode predictionunit of FIG. 3;

FIG. 5 is a diagram showing an example of in-screen prediction methodsfor an encoding operation performed by the intra predicting unit of FIG.3;

FIG. 6 is a schematic block diagram showing an example structure of animage decoding apparatus according to an embodiment of the presentinvention;

FIG. 7 is a diagram showing an example of the intra prediction unit ofFIG. 6;

FIG. 8 shows a detailed configuration of the LMchroma mode predictionunit of FIG. 7; and

FIG. 9 is a diagram showing an example of in-screen prediction methodsfor a decoding operation performed by the intra predicting unit of FIG.7.

MODE OF THE INVENTION

The attached drawings for illustrating preferred embodiments of thepresent invention are referred to in order to gain a sufficientunderstanding of the present invention, the merits thereof, and theobjectives accomplished by the implementation of the present invention.

Hereinafter, the present invention will be described in detail byexplaining preferred embodiments of the invention with reference to theattached drawings. Like reference numerals in the drawings denote likeelements.

Throughout the specification, when a portion “includes” an element,another element may be further included, rather than excluding theexistence of the other element, unless otherwise described. As usedherein, the terms “unit”, “element”, “module”, etc. should be understoodas units in which at least one function or operation is performed andwhich may be embodied in the form of hardware, software, or acombination of hardware and software.

As described above, an extended chroma mode (ECM) is a technique forpredicting pixel values in a chroma area with reference to an encodedluma area during an intra prediction for chroma pixel. By using thetechnique, a very high coding gain may be obtained with respect to a RGBimage exhibiting high correlations between color planes. The ECM isincluded to an intra prediction mode for chroma pixel and is configuredto be selected via a competition with conventional intra predictionmodes for chroma pixel.

Since a RGB image is encoded in the order of G, B, and R, a color planeB and a color plane R may be encoded in the intra prediction mode forchroma pixel. Equation 1 below is a mathematical expression of the ECMtechnique regarding a RGB image.C _(pred)[x,y]αG _(recon)[x,y]+β  [Equation 1]

Here, C_(pred) denotes a pixel value of a B block or a R block to bepredicted, G_(recon) denotes a decoded pixel value of anencoding-completed G block, and [x, y] denote locations of pixels. α andβ are weights and are inferred by using pixel values of the G block atsame locations as pixel values around the B block or the R block.

As shown in Equation 1, the ECM is a technique for predicting chromapixel values by using pixel values of an encoded luma pixel. However,the ECM technique may be useful only when correlations between colorplanes are high.

FIG. 1 is a diagram showing intra correlations between high frequencyareas of color planes in an image.

FIG. 1 shows intra correlations between the high frequency areas HH ofcolor planes G-B and color planes G-R in the first frame of a VenueVuimage from among the eight images shown in Table 1. FIG. 1A shows anoriginal image, FIG. 1B shows intra correlations between the highfrequency areas of the color planes G-B, and FIG. 1C shows intracorrelation between the high frequency areas of the color planes G-R. InFIGS. 1B and 1C, the correlations between the color planes G-B and thecorrelations between the color planes G-R are calculated by splittingscreen images to 8×8 blocks. Darker areas indicate lower correlations,whereas brighter areas indicate higher correlations.

Referring to FIGS. 1B and 1C, an image includes both areas correspondingto high correlations between high frequency areas of color planes andareas corresponding to low correlations between the high frequency areasof the color planes.

Therefore, according to the present invention, when pixel values arepredicted by using the ECM to improve accuracy of a prediction betweencolor planes, efficiency of the prediction between the color planes maybe improved by removing high frequency ingredients of a luma (G) areahaving a low correlation in advance. Furthermore, to handle changes ofcorrelations of an intra high frequency area, the present inventionsuggests a prediction unit (PU) adaptive filtering technique.

FIG. 2 is a schematic block diagram showing an image encoding apparatus100 according to the present invention.

Referring to FIG. 2, the image encoding apparatus 100 includes a motionpredicting unit 111, a motion compensating unit 112, an intra predictingunit 120, a switch 115, a subtractor 125, a transforming unit 130, aquantizing unit 140, an entropy encoding unit 150, an inverse quantizingunit 160, an inverse transforming unit 170, an adder 175, a filter unit180, and a reference image buffer 190.

The image encoding apparatus 100 encodes an input image in an intra modeor an inter mode and outputs a bitstream. In the below embodiments ofthe present invention, an intra prediction may be used as a synonym ofan in-screen prediction, whereas an inter prediction may be used as asynonym of an inter-screen prediction. To determine the optimalprediction method regarding a prediction unit, an intra prediction modeand an inter prediction mode may be selectively applied with respect tothe prediction unit. The image encoding apparatus 100 may generate aprediction block regarding an original block of an input image andencodes a difference between the original block and the predictionblock.

In an intra prediction mode, the intra predicting unit 120 (an intraprediction unit may be used as a synonym) supports a chroma mode forpredicting chroma pixel values (B, R) with reference to pixel values ofan encoded Luma (G) pixel during intra predictions of chroma (B, R)screen images. In particular, the intra predicting unit 120 according tothe present invention provides not only chroma modes provided by aconventional intra prediction unit, which include a planar mode, avertical mode, a horizontal mode, and a DC mode, but also an LMchromamode, which is an ECM suggested by J. Kim. However, the intra predictingunit 120 according to the present invention may calculate a correlationbetween chroma (B, R) pixels adjacent to a Luma (G) pixel when aprediction block is generated, and remove high frequency region of Luma(G) pixel by using low pass filter (LPF) based on the calculatedcorrelation, and then apply LMchroma mode to the Luma (G) pixel. Inother words, the intra predicting unit 120 may perform low passfiltering to Luma (G) pixel based on a correlation between the Chromaareas (B, R) and the Luma (G) area, which are adjacent to each other,before applying the LMChroma mode to Luma (G) pixel.

Furthermore, the intra predicting unit 120 generates prediction blocksin all available modes, selects the most efficient mode among the allavailable modes, and outputs a prediction block generated in theselected mode. Detailed descriptions of the intra predicting unit 120will be given below.

In an inter prediction mode, the motion predicting unit 111 searches foran area of a reference image stored in the reference image buffer 190,which is the best-matched to an input block, and calculates a motionvector during a motion prediction. The motion compensating unit 112generates a prediction block by performing motion compensation by usingthe motion vector.

The subtractor 125 generates a residual block based on a differencebetween the input block and the generated prediction block. Thetransforming unit 130 performs transformation with respect to theresidual block and outputs a transformation coefficient. Furthermore,the quantizing unit 140 quantizes the input transformation coefficientaccording to quantization parameters and outputs a quantizedcoefficient. The entropy encoding unit 150 entropy-encodes the inputquantized coefficient based on probability distribution and outputs abitstream.

Since inter prediction encoding, that is, inter-screen predictionencoding is performed in the HEVC, it is necessary to decode and store acurrently encoded image to be used as a reference image. Therefore, aquantized coefficient is inverse quantized by the inverse quantizingunit 160 and is inverse transformed by the inverse transforming unit170. The inverse-quantized and inverse-transformed coefficient is addedto a prediction block by the adder 175, and thus a restored block isgenerated.

The reconstructed block is filtered by the filter unit 180, where thefilter unit 180 may apply at least one of a deblocking filter, a sampleadaptive offset (SAO), and an adaptive loop filter (ALF) to thereconstructed block or a reconstructed picture. The filter unit 180 mayalso be referred to as an adaptive in-loop filter. The deblocking filtermay remove block distortions formed at boundaries between blocks. TheSAO may add a suitable offset value to a pixel value for compensating acoding error. The ALF may be performed based on values obtained bycomparing a reconstructed image to an original image and may beperformed only when high efficiency is required. The reconstructed blockfiltered by the filter unit 180 is stored in the reference image buffer190.

FIG. 3 is a diagram showing an example of the intra prediction unit ofFIG. 2.

In the HEVC, in addition to encoding of macro block in the related art,three new units, which are a coding unit (CU), a prediction unit (PU),and a transformation unit (TU), are additionally defined. From among thenew units, when a block-by-block splitting operation is performed andcoding units are determined, the prediction unit is determined withrespect to all coding units that are no longer split. According to thepresent invention, when the intra predicting unit 120 predicts a chromapixel value (B, R) via prediction between color planes during predictionof pixel values by using prediction units, high frequency ingredients ofa luma (G) pixel that is arranged at a same location as chroma (B, R)pixels to be encoded are removed by using a low pass filter (LPF).Therefore, the present invention may provide a filtering techniqueadaptive to changes of correlations of a luma (G) pixel from which highfrequency regions are removed.

Although the intra predicting unit 120 of FIG. 3 may support modes otherthan the chroma mode, only the configuration regarding the chroma modefor predicting chroma (B, R) pixel values with reference to encoded luma(G) pixel values is shown for convenience of explanation.

In FIG. 3, the intra predicting unit 120 includes a chroma predictingunit 121, a rate-distortion optimizing unit 122, and a mode selectingunit 123.

Like a conventional intra prediction unit, the chroma predicting unit121 includes a planar mode prediction unit PM, a vertical modeprediction unit VM, a horizontal mode prediction unit HM, and a DC modeprediction unit DC mode. The planar mode prediction unit PM, thevertical mode prediction unit VM, the horizontal mode prediction unitHM, and the DC mode prediction unit DC mode perform intra predictions byusing intra prediction modes known in the art, that is, a planar mode, avertical mode, a horizontal mode, and a DC mode. Since those predictionsare known in the art, detailed descriptions thereof will be omitted.

Furthermore, the chroma predicting unit 121 according to the presentinvention further includes a LMchroma mode prediction unit LMCM for theECM.

When the chroma predicting unit 121 performs chroma predictions in allmodes set up by using the planar mode prediction unit PM, the verticalmode prediction unit VM, the horizontal mode prediction unit HM, the DCmode prediction unit DC mode, and the LMchroma mode prediction unitLMCM, the rate-distortion optimizing unit 122 receives pixel values ofprediction blocks chroma-predicted in a plurality of modes anddetermines an optimal mode.

The mode selecting unit 123 receives mode information regarding theoptimal mode determined by the rate-distortion optimizing unit 122, setsup a flag corresponding to the optimal mode, and transmits the set upflag with predicted pixel values regarding a corresponding block.

FIG. 4 shows a detailed configuration of the LMchroma mode predictionunit LMCM of FIG. 3.

In FIG. 4, the LMchroma mode prediction unit LMCM includes a correlationcalculating unit COC, a low pass filter LPF, and an extended chromaprediction unit ECM. The correlation calculating unit COC receives anencoded block including Luma (G) pixel and calculates a correlationbetween the Luma (G) pixel and the Chroma (B, R) pixel corresponding tothe Luma (G) pixel. Next, it is determined whether the calculatedcorrelation is smaller than a pre-set threshold value. If the calculatedcorrelation is equal to or greater than the pre-set threshold value, thecorrelation calculating unit COC directly transmits the block includingLuma (G) pixel to the extended chroma prediction unit ECM. If thecalculated correlation is smaller than the pre-set threshold value, thecorrelation calculating unit COC transmits the block including Luma (G)pixel to the low pass filter LPF.

The low pass filter LPF receives the block including Luma (G) pixel,removes high frequency region of the Luma (G) pixel, and transmits thelow-pass filtered Luma (G) pixel values to the extended chromaprediction unit ECM.

The extended chroma prediction unit ECM predicts Chroma (B, R) pixelvalues by applying the Luma (G) pixel values transmitted from thecorrelation calculating unit COC or the low pass filter LPF to Equation1.

Therefore, the LMchroma mode prediction unit LMCM off FIG. 4 may operatein the same regard as a LMchroma mode in the related art. However, if acorrelation between the Luma (G) area and the Chroma (B, R) areas issmaller than a threshold value, high frequency ingredients is removedfrom the Luma (G) pixel by applying the low pass filter LPF and theChroma (B, R) pixel values are predicted by performing an LMchroma mode.In the present invention, a technique for removing high frequencyingredients from the Luma (G) area block by applying the low pass filterLPF and predicting the Chroma (B, R) pixel values by performing anLMchroma mode is referred to as a LPF LMchroma mode.

Equation 2 is a mathematical expression of a block prediction techniqueperformed by the LPF LMchroma mode LPFM.C _(pred)[x,y]=αG _(recon) ^(lpf)[x,y]+β  [Equation 2](here, G_(recon) ^(lpf) denotes a pixel value of a low pass filtered Gblock, and, like in Equation 1, α and β are weights and are inferred byusing pixel values of the G block at same locations as pixel valuesaround the B block or the R block.)G _(recon) ^(lpf)[x,y]=0.5G _(recon)[x,y]+0.125G _(recon)[x+1,y]+0.125G_(recon)[x−1,y]+0.125G _(recon)[x,y+1]+0.125G _(recon)[x,y−1]  [Equation3]

Equation 3 is an example of calculating a pixel value G_(recon) ^(lpf)of a low pass filtered G block by using four neighboring pixel values.

The intra predicting unit 120 may provide not only conventional intraprediction modes, which are a planar mode, a vertical mode, a horizontalmode, and a DC mode, but also an LMchroma mode, which is an ECMsuggested by J. Kim, and a low pass filter (LPF) LMchroma mode, which isa mode for applying an ECM after removing high frequency ingredients ofa Luma (G) area by using a LPF based on a correlation between the Luma(G) area and the Chroma (B, R) areas even in the LMchroma mode accordingto the present invention. Furthermore, after block prediction isindividually performed in each of the modes, an optimal mode isselected, and information regarding the selected optimal mode istransmitted together with pixel values regarding a block that arepredicted in the optimal mode. Therefore, efficiency of predicting pixelvalues between color planes may be significantly improved.

Table 2 shows an example of flags according to an intra chromaprediction mode according to the present invention.

TABLE 2 chroma_pred_from _luma_enabled_flag = 1 chroma_pred_from_luma_enabled_flag = 0 intra_chroma_pred_mode prefix suffix prefixsuffix 6 (DM_CHROMA) 0 n/a n/a n/a 5 (proposed method) 10 1 0 n/a 4(ECM) 10 0 0 n/a 0 (PLANAR) 11 00 1 00 1 (VERTICAL) 11 01 1 01 2(HORIZONTAL) 11 10 1 10 3 (DC) 11 11 1 11

As shown in Table 2, as an intra chroma prediction mode indicates anLMchroma mode and a LPF LMchroma mode prediction unit may be performedeven in an LMchroma mode, a flag may be set up by adding a suffix 0(when filtering is not applied) or a suffix 1 (when filtering isapplied) based on whether a low pass filtering is performed in theLMchroma mode (prefix: 10) in case of binarization of intra predictionas shown in the second and third columns of Table 2. Although only theprefix ‘10’ bit was used as a flag for encoding a mode in an ECM mode inthe related art, in the present invention, both the prefix ‘10’ bit andthe suffix 1 bit are used to additionally use the LPF LMchroma mode,where the suffix 1 is used as a correlation flag. However, set locationsand values of flags may vary.

FIG. 5 is a diagram showing an example of intra prediction methods foran encoding operation performed by the intra predicting unit of FIG. 3.

To describe the intra prediction method of FIG. 5 with reference to FIG.3, the intra predicting unit 120 first analyzes a prediction mode set tothe image encoding apparatus 100 (operation S110). Since the imageencoding apparatus 100 may provide an inter prediction mode and an intraprediction mode as prediction modes as described above, the predictionmode of the image encoding apparatus 100 should be set to an intraprediction mode for activation of the intra predicting unit 120.Therefore, the intra predicting unit 120 first analyzes the setting ofthe image encoding apparatus 100. Next, it is determined as a result ofthe analysis whether a set prediction mode is the intra prediction mode(operation S120). If the set prediction mode is not an intra predictionmode (that is, the set prediction mode is an inter prediction mode), theinter prediction mode is performed. Since an inter prediction mode isnot a task performed by the intra predicting unit 120, the intrapredicting unit 120 does not perform any operation. However, if the setprediction mode is an intra prediction mode, the chroma predicting unit121 of the intra predicting unit 120 performs block predictions by usingall available prediction modes (operation S130). In FIG. 5, according tothe configuration of the chroma predicting unit 121 of FIG. 3, each offive prediction units, which are the planar mode prediction unit PM, thevertical mode prediction unit VM, the horizontal mode prediction unitHM, the DC mode prediction unit DC mode, and the LMchroma modeprediction unit LMCM, generates prediction blocks in a pre-setcorresponding mode. In particular, the correlation calculating unit COCof the LMchroma mode prediction unit LMCM calculates a correlationbetween the Luma (G) area and the Chroma (B, R) areas (operation S135).Next, it is determined whether the calculated correlation is smallerthan a pre-set threshold value (operation S136). If the calculatedcorrelation is smaller than the pre-set threshold value, a Luma (G) areablock is transmitted to the low pass filter LPF, where the low passfilter LPF performs a low-pass filtering with regard to the Luma (G)area block (operation S137). Next, the low-pass filtered Luma (G) areablock is transmitted to the extended chroma prediction unit ECM andprediction blocks regarding the Chroma (B, R) pixel are generated byusing an ECM technique (operation S137). However, if the calculatedcorrelation is equal to or greater than the pre-set threshold value, theLuma (G) area block is directly transmitted to the extended chromaprediction unit ECM, where the extended chroma prediction unit ECMgenerates prediction blocks including the Chroma (B, R) pixels byapplying an ECM technique with regard to the Luma (G) area block that isnot low-pass filtered.

Furthermore, the generated prediction blocks are transmitted to therate-distortion optimizing unit 122, and the rate-distortion optimizingunit 122 analyzes pixel values of the prediction blocks and determine anoptimal mode (operation S140). When the optimal mode is determined bythe rate-distortion optimizing unit 122, the mode selecting unit 123selects the determined optimal mode (operation S150). Next, a flag isset up according to pre-set rules based on the selected optimal mode(operation S160). When a flag is set up, the mode selecting unit 123outputs the set up flag together with a selected prediction block(operation S170).

FIG. 6 is a schematic block diagram showing an example structure of animage decoding apparatus according to an embodiment of the presentinvention.

Referring to FIG. 6, an image decoding apparatus 200 includes an entropydecoding unit 210, an inverse quantizing unit 220, an inversetransforming unit 230, an intra predicting unit 240, a motioncompensating unit 250, a filter unit 260, and a reference image buffer270.

The image decoding apparatus 200 receives a bitstream output by anencoding apparatus, decodes the bitstream in an intra mode or an intermode, and outputs a reconstructed image. In an intra mode, a predictionblock is generated by using an in-screen prediction mode. In an intermode, a prediction block is generated by using an inter-screenprediction mode. The image decoding apparatus 200 obtains a residualblock from an input bitstream, generates a prediction block, andgenerates a block reconstructed by adding the residual block to theprediction block, that is, a reconstructed block.

The entropy decoding unit 210 decodes the input bitstream based onprobability distribution and outputs a quantized coefficient. Thequantized coefficient is inverse quantized by the inverse quantizingunit 220 and is inverse transformed by the inverse transforming unit230. As a result of the inverse quantization and the inversetransformation of the quantized coefficient, a residual block isgenerated.

In an intra prediction mode, the intra predicting unit 240 generates aprediction block by performing a spatial prediction by using pixelvalues of blocks neighboring a current block. Here, in particular, theintra predicting unit 240 according to the present invention analyzesthe flag of an input bitstream and, if the flag is set to the LMchromamode, it is determined whether the correlation flag is set to the LPFLMchroma mode. If it is determined that the correlation flag is set tothe LPF LMchroma mode, high frequency ingredients of thedecoding-completed luma (G) area is removed by using a low pass filter(LPF) first. Next, an ECM technique for predicting chroma (B, R) pixelswith reference to the Luma (G) pixel from which the high frequencyingredients are removed is applied.

In an inter prediction mode, the motion compensating unit 250 generatesa prediction block by performing motion compensation by using a motionvector and a reference image stored in the reference image buffer 270.

The residual block and the prediction block are combined by the adder255, and the combined block is processed by the filter unit 260. Thefilter unit 260 may apply at least one of a deblocking filter, a SAO,and an ALF to a restored block or a restored picture. The filter unit260 outputs a reconstructed image. The reconstructed image is stored inthe reference image buffer 270 and may be used for an inter prediction.

FIG. 7 is a diagram showing an example of detailed configurations of anintra prediction unit.

An intra predicting unit 240 of FIG. 7 includes a flag analyzing unit241, a mode selecting unit 242, and a chroma predicting unit 243.

The flag analyzing unit 241 analyzes a flag included in a bitstream andtransmits information regarding an analyzed mode to the mode selectingunit 242. The mode selecting unit 242 selects one of a planar modeprediction unit PM, a vertical mode prediction unit VM, a horizontalmode prediction unit HM, a DC mode prediction unit DC mode, and aLMchroma mode prediction unit LMCM included in the chroma predictingunit 243 based on the mode analyzed by the flag analyzing unit 241,activates the selected prediction unit, and transmits the receivedbitstream to the activated prediction unit.

The chroma predicting unit 243 includes the planar mode prediction unitPM, the vertical mode prediction unit VM, the horizontal mode predictionunit HM, the DC mode prediction unit DC mode, and the LMchroma modeprediction unit LMCM, each of which is activated according to aselection of the mode selecting unit 242 and generates a predictionblock according to pre-set rules.

In particular, the LMchroma mode LMCM may predict the Chroma (B, R)pixel values with direct reference to the luma (G) pixel values based onthe correlation flag. Alternatively, the LMchroma mode LPFM may operatein the LPF LMchroma mode for removing high frequency ingredients of thedecoded luma (G) area by using a low pass filter (LPF) and predictingChroma (B, R) pixel values with reference to the Luma (G) pixel valuesfrom which the high frequency ingredients are removed.

Table 3 shows a result of analyzing performance of a LPF LM chroma modeaccording to the present invention by applying the LPF LM chroma mode tofirst frames of the eight RGB experimental images of Table 1. Table 3was obtained by using the HM10.0_RExt2.0 program, where the HEVC rangeextension common test conditions (D. Flynn, K. Sharman, “Common testconditions and software reference configurations for HEVC rangeextensions”, JCTVC-L1006, 12th Meeting, Geneva, Switzerland, January2013.) were applied for setting experiment variables. Furthermore,performance of the suggested algorithm technique has been tested inAll-Intra (AI) mode by using 8 RGB 4:4:4 test images, where a LPF usedin the test was 2D-separatble LPF (ho=[1 2 1]/4). As shown in Table 3,compared to the conventional LM chroma mode, when an intra chromaprediction mode according to the present invention was performed,average bitrate-gains of 0.4%, 0.6%, and 0.8% could be obtained atrespective color planes G, B, and R at the All Intra HE Super-High-tiercompared to the ECM, which is the conventional LM chroma mode.Therefore, an overall average bitrate-gain of 0.6% could be obtained.Furthermore, encoding speed was increased by about 8%, whereas decodingspeed was increased by 1%. In particular, since the decoding speed(complexity), which is most sensitive to speed increase, an averagebitrate-gain of 0.6% could be obtained without increasing encoding anddecoding speeds according to the technique according to the presentinvention. Therefore, the technique according to the present inventionmay be considered as an efficient technique.

TABLE 3 All Intra HE Super-High-tier G B R Traffic 0.0% −0.2% −0.6%Kimono1 −0.6% −0.9% −0.9% EBULupoCandlelight −0.5% −0.7% −0.9%EBURainFruits 0.6% 0.0% −0.5% VenueVu −0.8% −1.3% −1.2% DucksAndLegs−0.5% −0.3% −0.1% OldTownCross −0.6% −0.8% −0.8% ParkScene −0.7% −0.9%−0.9% Overall −0.4% −0.6% −0.8% Enc Time[%]  108% Dec Time[%]  101%

FIG. 8 shows a detailed configuration of the LM chroma mode predictionunit LMCM of FIG. 7.

In FIG. 8, the LM chroma mode prediction unit LMCM includes acorrelation flag determining unit CFA, a low pass filter LPF, and anextended chroma prediction unit ECM. The correlation flag determiningunit CFA receives an encoded block including Luma (G) pixels anddetermines whether a correlation flag is set to a flag analyzed by theflag analyzing unit 241. If no correlation flag is set, the Luma (G)pixels are directly transmitted to the extended chroma prediction unitECM. If a correlation flag is set, the Luma (G) pixels are transmittedto the low pass filter LPF.

The low pass filter LPF receives the Luma (G) area block, removes highfrequency ingredients of the Luma (G) area block, and transmits thelow-pass filtered pixel values to the extended chroma prediction unitECM.

The extended chroma prediction unit ECM predicts the Chroma (B, R) pixelvalues by applying the Luma (G) pixel values transmitted from thecorrelation flag determining unit CFA or the low pass filter LPF toEquation 1.

Therefore, the LMchroma mode prediction unit LMCM off FIG. 8 may operatein the same regard as a LMchroma mode in the related art based on a setvalue of the correlation flag. Alternatively, high frequency ingredientsmay be removed from the Luma (G) area by applying the low pass filterLPF and pixel values may be predicted by performing an LMchroma mode.

Although it is described above that the correlation flag determiningunit CFA is included in the LMchroma mode prediction unit LMCM forconvenience of explanation, the correlation flag determining unit CFAmay be included in the flag analyzing unit 241. If the correlation flagdetermining unit CFA is included in the flag analyzing unit 241, themode selecting unit 242 may transmit a Luma (G) area block to either thelow pass filter LPF or the extended chroma prediction unit ECM in theLMchroma mode prediction unit LMCM based on a correlation flag analyzedby the flag analyzing unit 241.

FIG. 9 is a diagram showing an example of intra prediction methods for adecoding operation performed by the intra predicting unit of FIG. 6.

To describe the intra prediction method of FIG. 9 with reference to FIG.7, the intra predicting unit 240 first analyzes a prediction mode set tothe image decoding apparatus 200 (operation S210). Next, it isdetermined whether the analyzed prediction mode is the intra predictionmode (operation S220). If the set prediction mode is not an intraprediction mode, the set prediction mode is an inter prediction mode,and thus the intra predicting unit 240 is not activated and the methodis terminated. However, if the set prediction mode is an intraprediction mode, a bitstream is received, and the flag of the receivedbitstream is analyzed (operation S230). It is determined through theanalysis of the flag whether the flag set to the bitstream indicates aLM chroma mode (operation S240). If the set flag does not indicate theLM chroma mode, a prediction block is generated in a mode correspondingto the set flag (operation S270). However, if the set flag is the LMchroma mode, it is determined whether a correlation flag is set thereto(operation S250). If it is determined that a correlation flag is set, alow pass filtering is performed with respect to a luma (G) area(operation S260). Next, prediction blocks regarding chroma (B, R) areasare generated by using the color plane of the low pass filtered luma (G)area (operation S270). However, if no correlation flag is set, low-passfiltering is not performed, and prediction blocks including Chroma (B,R) pixels are generated by using the color plane of the Luma (G) area(operation S270). Next, the generated prediction blocks are output(operation S280).

As described above, according to methods, apparatuses, and systems forencoding and decoding an image according to the present invention, in anECM for predicting Chroma (B, R) pixel with reference to a Luma (G)pixel during an intra prediction, an LM Chroma mode may apply an ECMtechnique after a low-pass filtering is selectively applied based on acorrelation between color planes, thereby improving coding efficiency.

The present invention can also be embodied as computer readable codes ona computer readable recording medium. The computer readable recordingmedium is any data storage device that can store data which can bethereafter read by a computer system. Examples of the computer readablerecording medium include read-only memory (ROM), random-access memory(RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storagedevices, etc. The computer readable recording medium can also bedistributed over network coupled computer systems so that the computerreadable code is stored and executed in a distributed fashion.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

The invention claimed is:
 1. An image decoding apparatus, comprising: anentropy-decoder configured to decode information specifying a LinearModel (LM) chroma prediction mode of a current block; and an intrapredictor configured to: obtain a predicted luma block of the currentblock; determine the LM chroma prediction mode among a plurality of LMchroma prediction modes specified by the decoded information; wherein:the plurality of LM chroma prediction modes comprises a first LM chromaprediction mode and a second LM chroma prediction mode, and both thefirst LM chroma prediction mode and the second LM chroma prediction moderepresent that a chroma prediction pixel in a chroma block of thecurrent block is derived by a luma value; obtain the luma value using aluma pixel corresponding to the chroma prediction pixel, the luma pixelbeing in a reconstructed luma block of the current block, and thereconstructed luma block being obtained based on the predicted lumablock; and generate the chroma prediction pixel using the luma valuemultiplied by a first weight, wherein the decoded information issignaled at a block level, wherein, under the first LM chroma predictionmode, the first weight is obtained by using a filtered luma pixel, thefiltered luma pixel being obtained by using luma pixels located to aleft of the reconstructed luma block, wherein, under the second LMchroma prediction mode, the first weight is obtained without using thefiltered luma pixel, wherein the luma value is obtained by further usingfour neighboring luma pixels adjacent to the luma pixel corresponding tothe chroma prediction pixel, wherein the luma value is obtained by aweighted sum of the luma pixel and the four neighboring luma pixels, andwherein a ratio between a first coefficient multiplied with the lumapixel and a second coefficient multiplied with the four neighboring lumapixels is 4:1.
 2. The apparatus for decoding the image of claim 1,wherein both of the first LM chroma prediction mode and the second LMchroma prediction mode are selectable for the current block regardlessof color format of a current picture including the current block.
 3. Theapparatus for decoding the image of claim 1, wherein the chromaprediction pixel is calculated by a formula as follows:C _(pred)[x,y]=α*L _(recon)+β ([x,y] is a location of the chromaprediction pixel, α is the first weight, β is a second weight, L_(recon)is the luma value, and C_(pred) is the chroma prediction pixel).
 4. Animage encoding apparatus, comprising: an intra predictor configured toobtain a predicted luma block of a current block; and an entropy-encoderconfigured to encode information at a block level, wherein theinformation specifies a Linear Model (LM) chroma prediction mode among aplurality of LM chroma prediction modes of the current block, whereinthe plurality of LM chroma prediction modes comprises a first LM chromaprediction mode and a second LM chroma prediction mode, wherein both theLM chroma prediction mode and the second LM chroma prediction moderepresent that a chroma prediction pixel in a chroma block of thecurrent block is derived by a luma value, wherein the luma value isobtained by using a luma pixel corresponding to the chroma predictionpixel, the luma pixel being in a reconstructed luma block of the currentblock, and the reconstructed luma block being obtained based on thepredicted luma block, wherein the chroma prediction pixel is generatedby using the luma value multiplied by a first weight, wherein, under thefirst LM chroma prediction mode, the first weight is obtained by using afiltered luma pixel, the filtered luma pixel being obtained by usingluma pixels located to a left of the reconstructed luma block, wherein,under the second LM chroma prediction mode, the first weight is obtainedwithout using the filtered luma pixel, wherein the luma value isobtained by further using four neighboring luma pixels adjacent to theluma pixel corresponding to the chroma prediction pixel, wherein theluma value is obtained by a weighted sum of the luma pixel and the fourneighboring luma pixels, and wherein a ratio between a first coefficientmultiplied with the luma pixel and a second coefficient multiplied withthe four neighboring luma pixels is 4:1.
 5. The apparatus for encodingthe image of claim 4, wherein both of the first LM chroma predictionmode and the second LM chroma prediction mode are selectable for thecurrent block regardless of color format of a current picture includingthe current block.
 6. The apparatus for encoding the image of claim 4,wherein the chroma prediction pixel is calculated by a formula asfollows:C _(pred)[x,y]=α*L _(recon)+β ([x,y] is a location of the chromaprediction pixel, α is the first weight, β is a second weight, L_(recon)is the luma value, and C_(pred) is the chroma prediction pixel).
 7. Anon-transitory computer-readable medium storing instructions that, whenexecuted by a processor, configure the processor to: decode informationspecifying a Linear Model (LM) chroma prediction mode of a currentblock; obtain a predicted luma block of the current block; obtain areconstructed luma block based on the predicted luma block, determinethe LM chroma prediction mode among a plurality of LM chroma predictionmodes specified by the decoded information, wherein: the plurality of LMchroma prediction modes comprises a first LM chroma prediction mode anda second LM chroma prediction mode, and both the first LM chromaprediction mode and the second LM chroma prediction mode represent thata chroma prediction pixel in a chroma block of the current block isderived by a luma value; obtain the luma value using a luma pixelcorresponding to the chroma prediction pixel, the luma pixel being inthe reconstructed luma block of the current block; and generate thechroma prediction pixel using the luma value multiplied by a firstweight, wherein the decoded information is signaled at a block level,wherein, under the first LM chroma prediction mode, the first weight isobtained by using a filtered luma pixel, the filtered luma pixel beingobtained by luma samples located to a left of the reconstructed lumablock, wherein, under the second LM chroma prediction mode, the firstweight is obtained without using the filtered luma pixel, wherein theluma value is obtained by further using four neighboring luma pixelsadjacent to the luma pixel corresponding to the chroma prediction pixel,wherein the luma value is obtained by a weighted sum of the luma pixeland four neighboring luma pixels, and wherein a ratio between a firstcoefficient multiplied with the luma pixel and a second coefficientmultiplied with the four neighboring luma pixels is 4:1.